Beamforming techniques are used in wireless communication systems increase throughput and/or density of cells.
Beamforming may be divided into transmission beamforming which is performed by the transmitting side and reception beamforming which is performed by the receiving side. Transmission beamforming generally uses a plurality of antennas and concentrates signals transmitted from respective antennas in a particular direction (that is, space), so as to increase directivity. A set of a plurality of antennas is referred to as an array antenna, and an antenna included in the array antenna is referred to as an antenna element or an array element. The antenna array may be configured in various types such as a linear array and a planar array. When the transmission beamforming is used, the distance of arrival can be increased through a signal directivity increase, and the signal is not transmitted in any direction other than the corresponding direction. As a result, interference influencing another user can be significantly reduced.
The receiving side may perform the reception beamforming by using a reception array antenna. The reception beamforming concentrates received radio waves to be directed in a particular direction, increases sensitivity of signals received from the particular direction, and excludes, signals received from another direction, so as to block interference signals.
For future standards, it is expected that Multiple-Input Multiple-Output (MIMO) precoding will be a typical way of implementing beamforming.
MIMO refers to a method using multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Namely, a plurality of antennas is used at a transmitter or a receiver of a wireless communication system so that capacity can be increased and performance can be improved. MIMO may also be referred to as multi-antenna in this disclosure.
MIMO technology does not depend on a single antenna path in order to receive a whole message. Instead, MIMO technology completes data by combining data fragments received via multiple antennas. The use of MIMO technology can increase data transmission rate within a cell area of a specific size or extend system coverage at a specific data transmission rate. MIMO technology can be widely used in mobile communication terminals and relay nodes. MIMO technology can overcome a limited transmission capacity encountered with the conventional single-antenna technology in mobile communication.
FIG. 1 illustrates the configuration of a typical MIMO communication system. A transmitter has NT transmit (Tx) antennas and a receiver has NR receive (Rx) antennas. Use of a plurality of antennas at both the transmitter and the receiver increases a theoretical channel transmission capacity, compared to the use of a plurality of antennas at only one of the transmitter and the receiver. Channel transmission capacity increases in proportion to the number of antennas. Therefore, transmission rate and frequency efficiency are increased. Given a maximum transmission rate Ro that may be achieved with a single antenna, the transmission rate may be increased, in theory, to the product of Ro and a transmission rate increase rate Ri in the case of multiple antennas, as indicated by Equation 1. Ri is the smaller of NT and NR.Ri=min(NT,NR)  [Equation 1]
For example, a MIMO communication system with four Tx antennas and four Rx antennas may theoretically achieve a transmission rate four times that of a single antenna system. Since the theoretical capacity increase of the MIMO wireless communication system was verified in the mid-1990s, many techniques have been actively developed to increase data transmission rate in real implementations. Some of these techniques have already been reflected in various wireless communication standards including standards for 3rd generation (3G) mobile communications, next-generation wireless local area networks, etc.
Active research up to now related to MIMO technology has focused upon a number of different aspects, including research into information theory related to MIMO communication capacity calculation in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of MIMO systems, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
Communication in a MIMO system will be described in detail through mathematical modeling. It is assumed that NT Tx antennas and NR Rx antennas are present as illustrated in FIG. 1. Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.s=[s1,s2, . . . ,sNT]T  [Equation 2]
Individual pieces of the transmission information s1, s2, . . . , sNT may have different transmit powers. If the individual transmit powers are denoted by P1, P2, . . . , PNT, respectively, then the transmission power-controlled transmission information may be given asŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Equation 3]
The transmission power-controlled transmission information vector ŝ may be expressed below, using a diagonal matrix P of transmission power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
Meanwhile, NT transmission signals x1, x2, . . . , xNT to be actually transmitted may be configured by multiplying the transmission power-controlled information vector Ŝ by a weight matrix W. The weight matrix W functions to appropriately distribute the transmission information to individual antennas according to transmission channel states, etc. The transmission signals x1, x2, . . . , xNT are represented as a vector X, which may be determined by Equation 5. Here, wij denotes a weight of an i-th Tx antenna and a j-th piece of information. W is referred to as a weight matrix or a precoding matrix.
                    x        =                                                       [                                                                                          x                      1                                                                                                                                  x                      2                                                                                                            ⋮                                                                                                              x                      i                                                                                                            ⋮                                                                                                              x                                              N                        T                                                                                                        ]                        =                                                            [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                            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                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                            =                                                W                  ⁢                                      s                    ^                                                  =                WPs                                                                        [                  Equation          ⁢                                          ⁢          5                ]            
Generally, the physical meaning of the rank of a channel matrix is the maximum number of different pieces of information that can be transmitted on a given channel. Therefore, the rank of a channel matrix is defined as the smaller of the number of independent rows and the number of independent columns in the channel matrix. Accordingly, the rank of the channel matrix is not larger than the number of rows or columns of the channel matrix. The rank of the channel matrix H (rank(H)) is restricted as follows.rank(H)≤min(NT,NR)  [Equation 6]
A different piece of information transmitted in MIMO is referred to as a transmission stream or stream. A stream may also be called a layer. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of different pieces of transmittable information. Thus, the channel matrix H is determined by# of streams≤rank(H)≤min(NT,NR)  [Equation 7]
“# of streams” denotes the number of streams. It should be noted that one stream may be transmitted through one or more antennas.
One or more streams may be mapped to a plurality of antennas in many ways. This method may be described as follows depending on MIMO schemes. If one stream is transmitted through a plurality of antennas, this may be regarded as spatial diversity. When a plurality of streams is transmitted through a plurality of antennas, this may be spatial multiplexing. A hybrid scheme of spatial diversity and spatial multiplexing may be contemplated.
Another aspect of future wireless communication standards is that users will expect to enjoy the services in vehicles, i.e. with significant relative speeds between the transmitter and the receiver. The relative speed gives rise to distortions of the radiofrequency spectrum due to the Doppler effect.
In conventional systems, the Doppler effect is considered detrimental and it is compensated by an adjustment of the frequency of local oscillators. Doppler shifts become larger as the carrier frequencies used by wireless systems are higher and higher.